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THE TEKLANIKA FORMATION - A NEW PALEOCENE VOLCANIC FORMATION IN
THE CENTRAL ALASKA RANGE
BY
Wyatt G. Gilbert, Virginia M. Ferrell, and Donald L. Turner
GEOLOGIC REPORT 47
STATE OF ALASKA
Jay S. Hammond, Governor
Guy R. Martin, Commissioner, Dept. o f Natural Resources
Ross G . Schaff, State Geologist
For sale by Alaska Division of Geological and Geophysical Surveys, P.O. Box 80007, College, 99708; 323 E. 4th Ave., Anchorage,
99501; P.O. Box 2438, Ketchikan, 99901; and Pouch M, Juneau, 99811
CONTENTS
Page
Abstract ....................................................................................................................................................................
Introduction .............................................................................................................................................................
Purpose ................................................................................................................................................................
Previous usage ......................................................................................................................................................
Present study .......................................................................................................................................................
General description of the Teklanika Formation ..................................................................................................
Related igneous rocks ...........................................................................................................................................
Volcanic stratigraphy and petrology .........................................................................................................................
Upper Teklanika River sequence.........................................................................................................................
Summary .........................................................................................................................................................
Unit A .............................................................................................................................................................
Unit B ..............................................................................................................................................................
Unit C ..............................................................................................................................................................
Unit D..............................................................................................................................................................
Unit E ..............................................................................................................................................................
Unit F ..............................................................................................................................................................
Volcanic rocks of Cathedral and Igloo Mountains ...............................................................................................
Volcanic rocks of Polychrome Mountain ..........................................................................................................
Volcanic rocks of .upper Toklat River ...................................................................................................................
Intrusive rocks ......................................................................................................................................................
Petrology ..............................................................................................................................................................
Felsic flows and intrusions...............................................................................................................................
Pyroclastic rocks ..............................................................................................................................................
Mafic and intermediate flows and intrusions ....................................................................................................
Geochemistry ............................................................................................................................................................
Geochronology ..........................................................................................................................................................
Acknowledgments.....................................................................................................................................................
References cited ........................................................................................................................................................
5
ILLUSTRATIONS
Page
Plate
Figure
1. Geologic map and structure sections of Teklanika River-Polychrome
Mountain area .................. In pocket
1. Distribution of Teklanika Formation and location o f study area ...........................................................
2 . Looking a t southwest side of Igloo Mountain ........................................................................................
3 . Lower part of upper Teklanikasequence o n east side of upper Teklanika River ...................................
4 . Looking north a t eastern part of Polychrome Mountain .......................................................................
5. Looking south a t east-dipping andesite and rhyolite flows o n east side o f upper Toklat River ..............
6 . Diabase plug o n east side of Cathedral Mountain ..................................................................................
7 . Photomicrograph of rhyolite flow from unit F of upper Teklanika sequence ........................................
8. Photomicrograph of rhyolite flow from Cathedral Mountain ................................................................
9 . Photomicrograph of devitrified welded tuff from Cathedral Mountain displaying highly compressed
shards and sanidine crystal fragments ................................................................................................
1 0. Photomicrograph o f devitrified welded tuff along lower Calico Creek ..................................................
'11. Photomicrograph of andesite flow from Polychrome Mountain ............................................................
1 2 Photomicrograph of andesite flow from unit E of upper Teklanika sequence .......................................
1 3. Chemical classification of nine volcanic rocks from Teklanika Formation after Irvine and
Baragar. 1 9 7 1....................................................................................................................................
1 4. Alkalies-silica diagram with nine volcanic rocks from Teklanika Formation plotted .............................
1 5. AFM diagram with nine volcanic rocks from Teklanika Formation plotted ..........................................
1 6. AlgOR
. ..Normative plagioclase diagram with nine volcanic rocks from Teklanika Formation
plotted ...............................................................................................................................................
.
CONTENTS
TABLES
Page
Table
1. Modes of selected rhyolite flows and felsic intrusions.......................................................................... 8
2 . Modes of selected andesite and basalt flows and mafic and intermediate intrusions.............................. 11
13
3 . Geochemical analyses......................................................................................................................
13
4 . Analytical data for K-Ar age determinations .....................................................................................
-
THE TEKLANIKA FORMATION A NEW PALEOCENE VOLCANIC FORMATION IN
THE CENTRAL ALASKA RANGE
By Wyatt G . Gilbert,l Virginia M. Ferrell,.' and Donald L. Turner2
ABSTRACT
PREVIOUS USAGE
A series of andesite, rhyolite, and basalt flows, felsic
pyroclastic rocks, and .related intrusive rocks covers
about 165 square kilometers in the eastern part of
Mount McKinley National Park. These rocks, once considered part of the Cantwell Formation, are now defined
as the Teklanika Formation. The formation, which
locally reaches a minimum thickness of 3,750 meters
overlies the Cantwell Formation both conformably and
unconformably. The type section of the Teklanika
Formation forms the ridge east of the upper Teklanika
River drainage and is composed of six conformable
units.
Felsic volcanic flows are generally amygdaloidal
porphyritic rhyolites with sanidine phenocrysts. Pyroclastic rocks are dominantly a combination of andesitic
to rhyolitic vitric, lithic, and crystal welded tuffs. Mafic
flows range from olivine basalt to andesite.
Chemically, the Teklanika Formation is part of a
calc-alkali series and may be cogenetic with early
Tertiary plutonic rocks in south-central Alaska.
Minimum radiometric ages of 60.6 and 41.8 m.y.,
together with an age of 57.2 2 4. m.y. from a related
plug, suggest that eruption of the Teklanika Formation
and orogeny occurred during Paleocene time.
The Cantwell Formation was first recognized by
Eldridge, who described the "Cantwell Conglomerate"
as being "...a series of conglomerates and coarse sandstones which was encountered in the banks of the
Cantwell River [Nenana River] about 1 0 or 15 miles
above the lower forks" (Eldridge, 1900, p. 16). The
next reference to the Cantwell Formation was by
Brooks (1911), who mapped and thoroughly described
the Cantwell Formation in the Mount McKinley National Park area and first included volcanic rocks as
part of the formation. He states: "In this area, especially toward the Nenana, lava flows interbedded with
the conglomerate are very prevalent. In some localities
these volcanic rocks form over half of the thickness of
strata exposed belonging to this formation. These lavas
appear to be chiefly andesites, but also include rhyolites
and basalts. That they were surface flows is indicated
both by their stratigraphic relations and by the amygdaloidal and columnar structure that some of them
exhibit. Some tuffaceous beds also occur in this formation" (Brooks, 1911, p. 79).
The "interbeds," which Brooks describes on the next
two pages of his report, are apparently metavolcanic
rocks and quartz latite interbeds in the Mount SheldonWyoming Hills area, which he probably mistakenly
correlated with the andesite and rhyolite flows to the
south (fig. 1). From 1911 onward, workers have
generally included the volcanic rocks described in this
paper as part of the Cantwell Formation (Capps, 1919,
1930, 1940; Reed, 1961; Wolfe and Wahrhaftig, 1970).
Our study of these rocks, however, shows that the
andesite-rhyolite flows lie above the sedimentary section
of the cantwell Formation, are in part unconformable
on it, and form a distinct lithologic contrast with the
sedimentary rocks of the Cantwell Formation. We do
not wish to imply that the Cantwell Formation contains
no volcanic interbeds. However, they are minor compared with sedimentary rocks of the Cantwell Formation; they also differ petrologically. The Cantwell
Formation, therefore, is herein restricted to the generalIY conformable sequence of sandstone, siltstone. and
conglomerate, as suggested originally by Eldridge (1900).
INTRODUCTION
PURPOSE
The intent of this report is to name, describe, and
radiometrically date a new volcanic formation-the
Teklanika Formation-in
the central Alaska Range,
Alaska. This formation has previously been included
in the Cantwell Formation. The Teklanika Formation
is named after the Teklanika River, a major tributary of
the Toklat River, where the formation is particularly
well exposed and where its thickest known section is
found. The report does not completely describe the
Teklanika Formation, however, because much of its
outcrop area remains unmapped in detail.
-.
~ D G G Sand University of Alaska geology department. College,
AK 99708.
of Alaska Geophysical Institute and geology department. College. AK 99108.
1
GEOLOGIC REPORT 47
THE TEKLANIKA FORMATION
3
PRESENT STUDY
This paper is the result of a 3-year study of the
geology of the Healy C-6 quadrangle and immediate
vicinity by Gilbert (Gilbert and Redman, 1975). In
addition, petrologic examination of many of the volcanic rocks by Ferrell and radiometric dating of three
rock specimens by Ferrell and Turner are included as an
important part of the study.
GENERAL DESCRIPTION OF
THE TEKLANIKA FORMATION
The Teklanika Formation is primarily composed of
flows of andesite and rhyolite, with lesser amounts of
basalt and pyroclastic rocks (igneous rock classification
after Peterson, 1961). The formation covers about
165 square kilometers in the upper Toklat, East Fork,
Teklanika, and Sanctuary River drainages in Mount
McKinley National 'Park (fig. I ) , of which about 85
square kilometers has been mapped at 1:40,000 scale
(Gilbert and Redman, 1975). In this area the Teklanika
Formation is in sharp conformable or unconformable
contact with the underlying sedimentary rocks of the
Cantwell Formation. The contact is commonly marked
by a 0.5- to 3.0-meter-thick light-colored tuff bed. On
Igloo and Cathedral Mountains, the formation conformably overlies a lenticular pebble-cobble-boulder
conglomerate with abundant sandstone and volcanic
clasts, and typical sandstone, siltstone, and conglomerate
of the Cantwell Formation (Gilbert and Redman, 1975)
(pl. 1, fig. 2). This upper Cantwell conglomerate
overlies a thick sequence of sandstone and siftstone and
suggests that uplift of a nearby volcanic and sedimentary
terrain took place concurrently with the beginning of
volcanism. On Polychrome Mountain, the gently folded
Teklanika Formation lies with apparent unconformity
above a tightly folded sequence of Cantwell sandstone
and conglomerate (pl. 1). Although this contact could
be due of different mechanical responses by volcanic
and sedimentary units during folding, no evidence of
mechanical detachment was observed at the base of the
volcanic sequence.
Basalt, andesite, and minor pyroclastic rocks that
cover 30 square kilometers and cap Mount Fellows in
the lower Yanert River drainage just east of Mount
McKinley National Park (Hickman, 1974) are here
considered part of the Teklanika Formation (fig. 1).
These rocks overlie the Cantwell Formation concordantly or with slight discordance (Hickman, 1974).
Quartz latite flows interbedded with conglomerate
in the Wyoming Hills (fig. 1 ) differ in petrology and
stratigraphic position from the Teklanika Formation
(Redman, 1974); they are not considered part of the
Teklanika Formation, but rather as interbeds within
the Cantwell Formation. Volcanic rocks in the Mount
Figure 2. Looking at southwest side of Igloo Mountain.
Tva, andesite and basalt flows; Tcu, upper conglomerate of Cantwell Fgrpation; Tc, Cantwell Formation.
Galen area (fig. 1 ) that were thought by Reed (1961) to
be part of the Cantwell Formation are now known to be
much younger (Decker, 1975). Hornblende andesite
flows and shallow intrusions on Polychrome Mountain
(fig. 1) are also younger than the Teklanika Formation.
RELATED IGNEOUS ROCKS
Paleocene calc-alkaline volcanism in the central Alaska Range is only a small part of the extensive orogenesis
and igneous activity that characterized the CanadianAlaskan Cordillera during late Mesozoic and early
Tertiary time (Lathram, 1973; Monger and others,
1972). In the Canadian Cordillera, extensive plutonism
and volcanism occurred during uppermost Cretaceous,
Paleocene, and Eocene time (Souther, 1967, Gabrielse,
1967).
In the western Aleutian Islands, major orogenesis
accompanied by andesitic volcanism also took place
during Late Cretaceous and early Tertiary time (Coats,
1962). In the eastern Aleutian Islands and the Alaska
Peninsula, early Tertiary deformation was accompanied
by andesitic 'volcanism and intrusion of granitic to
quartz diorite batholiths (Burk, 1965).
In the northern Aleutian Range and southern and
central Alaska Range, three episodes of intrusive activity during Jurassic, Late Cretaceous and early Tertiary, and mid-Tertiary time were accompanied by
related volcanism (Reed and Lanphere, 1973). In the
southern and central Alaska Range, Late Cretaceous
and early Tertiary intrusions range mainly from quartz
diorite to granite (Reed and Lanphere, 1973). The
Teklanika Formation of the central Alaska Range is a
series of calc-alkaline volcanic flows and tuff beds which
are, perhaps, the surface expression of this Late Cretaceous--early Tertiary intrusive epoch.
4
GEOLOGIC REPORT 47
VOLCANIC STRATIGRAPHY AND PETROLOGY
Within the study area the thickest known section of
the Teklanika Formation underlies the ridge dividing
the Calico Creek and the upper Teklanika River drainages (pl. 1). Other extensive exposures of the formation
are found in the Cathedral-Igloo Mountains area, on the
east side of Polychrome Mountain, and along the upper
Toklat River. Coeval dikes, sills, and small stocks are
scattered throughout this volcanic province. Complex
faulting, overlying late Cenozoic sedimentary rocks,
and extreme variation in volcanic stratigraphy make
correlation between major areas of volcanic outcrop
difficult. The stratigraphy of four representative areas
will be described separately. The petrology of different
parts of the formation is similar and will be discussed
in one section (p. 7).
UPPER TEKLANIKA RIVER SEQUENCE
SUMMARY
This sequence forms the most complete known
section of volcanic flows and is designated the type
section of the Teklanika Formation (pl. 1, fig. 3). Six
conformable units (units A-F) with a minimum thickness
of approximately 3,750 meters compose the sequence.
The base of the upper Teklanika River sequence is
6-112 kilometers south of Calico Creek along the upper
Teklanika River. Here the sequence is in probable fault
contact with the Cantwell Formation. The top of the
sequence is 5 kilometers south of Calico Creek along the
east side of the Teklanika River, where the upper unit
in the sequence is unconformably overlain by Quaternary deposits.
UNIT A
The lower part of the basal unit of the upper
Teklanika River sequence is a series of dark-gray
porphyritic and aphanitic basalt flows that form rugged
topography on both sides of the upper Teklanika River.
Individual flows range from 20 to 40 meters thick and
are commonly jointed and fractured. In hand specimen,
the porphyritic flows contain fine- to medium-grained
phenocrysts of pyroxene and hornblende, altered t o
chlorite and set in an altered groundmass containing
abundant plagioclase. This series of basalt flows exhibits the highest degree of deuteric alteration of any
rocks in the formation. The thickness of this series of
flows is approximately 1,375 meters.
Overlying the series of basalt flows is an interval of
interbedded mudstone, tuff, tuff breccia, and pillow
basalt about 50 to 60 meters thick. These beds are
present on both sides of the upper Teklanika River.
The lowest bed in the interval is a 5-meter-thick darkgray to black carbonaceous mudstone that contains
pieces of wood and plant material. For 20 meters above
the dark mudstone bed, thin mudstone beds 0.1 to
0.3 meters thick are interbedded with light-gray to
white water-laid rhyolite tuff and tuff breccia and
gray to black tuff beds. The top of the interval is
composed of about 30 meters of gray-green pillow
basalt.
Figure 3. Lower part of upper Teklanika River sequence on east side of upper Teklanika River. Tva-a - u n i t A, Tvf-b unit B, Tvf-c - unit C of upper Teklanika River sequence; Tc - Cantwell Formation, Tia - mafic intrusion.
THE TEKLANIKA FORMATION
UNIT B
In this unit about 4 0 resistant flows of brown- and
purple-weathering gray andesite, each 2 t o 5 meters
thick, alternate with beds o f less resistant tuff and tuff
breccia, 5 t o 1 0 meters thick (fig. 3). The unit is about
400 meters thick.
UNIT C
This unit is composed of a rugged series of purpleweathering light-gray microcrystalline rhyolite flows 20
t o 5 0 meters thick. The rhyolite flows are similar in
lithology t o flows in unit B, but differ in thickness and
also lack interbedded tuff and tuff breccia. The unit is
from 545 meters to 6 0 0 meters thick. West of the upper
Teklanika River, units B and C merge into a composite
unit with beds of tuff and tuff breccia scattered throughout.
UNIT D
This unit, composed of light-tan vesicular porphyroaphanitic rhyolite flows 5 t o 5 0 meters thick, is exposed
on both the east and west sides of the upper Teklanika
River. Interbeds of amygdaloidal olivine-augite basalt
and andesite, 5 t o 1 0 meters thick and locally displaying
columnar jointing, are common in this unit. Scattered
beds of rhyolite crystal tuff are also present. The unit
is about 600 meters thick.
UNIT E
This unit contains a series of dark-brown-weathering
dark-gray olivine-augite basalt and andesite flows, 5 t o
4 0 meters thick. The tops of the flows are commonly
amygdaloidal, with vesicles filled with calcite, chalcedony, o r quartz. Interbeds of dark-gray agglomerate,
containing clasts of mafic and intermediate volcanic
rocks set in a dark-gray vesicular glassy groundmass,
commonly overlie the flows. Beds of rhyolite tuff
and tuff breccia 1 t o 2 meters thick are intermittently
interbedded with the basalt and andesite flows. Scattered flows of magnetite-rich basalt crop o u t east of the
upper Teklanika River. The unit is from 430 t o 670
meters thick.
UNIT F
This unit is composed of a thick flow of light-brownweathering light-gray porphyritic rhyolite with sanidine
phenocrysts. The base of the unit is marked by a 2meter-thick black-and-white bed of obsidian. The unit
has a minimum thickness of 60 meters and is unconformably overlain by Quaternary alluvial deposits.
VOLCANIC ROCKS O F CATHEDRAL
AND IGLOO MOUNTAINS
A thick series of volcanic rocks caps Igloo Mountain,
is exposed along Tattler Creek, and crops out over
most of Cathedral Mountain (pl. 1). Two volcanic
units are present: a lower andesite unit and an upper
rhyolite unit. A major northeast-trending fault on
Cathedral Mountain repeats the two units. Small intrusive dikes and sills are found intruding the extrusive
rocks.
The andesite unit is present on both Cathedral and
Igloo Mountains, where it conformably overlies the
Cantwell Formation. On Igloo Mountain, about 1 2
brown-weathering gray aphanitic vesicular andesite flows
are interbedded with minor laterally discontinuous tuff
beds (fig. 2). The tuff beds are 0.5 to 1.0 meters
thick, light green, and vesicular. On the northeast side
of Igloo Mountain, a 2-meter-thick light-gray tuff bed
marks the contact between the Teklanika and Cantwell
Formations.
On the west side of Cathedral Mountain and in
Tattler Creek, andesite and basalt flows are gray t o
black, aphanitic t o fine grained, and vesicular. In this
area, basalt flows are common in the lower portion of
the unit. The andesite and basalt flo,ws weather reddish
brown and commonly exhibit columnar jointing. The
contact with the underlying Cantwell Formation is
marked by interbedded tuffaceous sedimentary rocks
and volcanic flows, and is placed a t the base of a 3meter-thick light-colored tuff bed. The upper part of
the andesite unit on Cathedral Mountain and along
Tattler Creek contains interbedded dark-weathering rhyolite flows, similar to those in the overlying rhyo1it.e
unit. The thickness of the andesite unit ranges from
500 meters on Cathedral Mountain to 770 meters on
Igloo Mountain.
A series of rhyolite flows and pyroclastic rocks
overlies the andesite unit south of Tattler Creek and on
Cathedral Mountain. The flows are siliceous microcrystalline t o hyaline rocks from 3 t o 25 meters thick.
The flows south of Tattler Creek and on the northern
and central parts of Cathedral Mountain are intruded
by many small dikes and sills. Rhyolite crystal-lithic
tuff and volcaniclastic lithic sandstone are found interbedded with the rhyolite flows on the central part of
Cathedral Mountain. The minimum thickness of the
rhyolite unit reaches 500 meters on Cathedral Mountain.
East of the Teklanika River and north of a major
east-west-trending fault 3 kilometers south of the mouth
of Calico Creek are two units that appear t o be in fault
contact with one another. The southern unit is made u p
of a variety of interbedded pyroclastic beds and rhyolite
flows that are 5 0 t o 6 0 meters thick. This unit has a
minimum thickness of about 1,100 meters. Common
lithologies observed include: andesitic lithic-crystal welcl-
6
GEOLOGIC REPORT 47
ed tuff, medium- to coarse-grained volcaniclastic lithic
sandstone with tuff and feldspar fragments, porphyroaphanitic rhyolite flows with altered microphenocrysts
of sanidine, and other cryptocrystalline to microcrystalline flows with fine- to medium-grained plagioclase
phenocrysts.
The northern unit along the lower part of Calico
Creek includes at least 1,500 meters of augite basalt and
pyroclastic rocks in beds 1 0 to 20 meters thick. The
pyroclastic rocks are in part composed of crystal-lithic
welded tuff, but the relative abundance of each rock
type is unknown. The relationship of these two units
to the volcanic rocks on Cathedral Mountain, or to the
upper Teklanika sequence to the south, is unclear.
VOLCANIC ROCKS OF POLYCHROME MOUNTAIN
Two series of rhyolite flows sandwich a sequence of
andesite flows on Polychrome Mountain (pl. 1). In both
series, flows of porphyro-aphanitic flow-banded rhyolite contain interbeds of rhyolite crystal tuff and
lithic welded tuff. Because scattered exposures of
rhyolite are found between Polychrome Mountain and
the rhyolite unit exposed along Tattler Creek, the
rhyolite flows on Polychrome Mountain may be continuous with the upper rhyolite unit on Cathedral
Mountain. The lower series of rhyolite flows forms
an angular unconformity with the underlying Cantwell
Formation and ranges in thickness from 0 tp 350
meters (fig. 4). The upper series of rhyolite flows has a
maximum exposed thickness of 750 meters.
Separating the two units of rhyolite flows is an
andesite unit consisting of about 1 0 flows, each from
3 to 30 meters thick. The flows become thinner in
the upper part of the unit. The flows are vesicular and
bear geodes containing hematite, chalcedony, and calcite
or quartz or both. The thickness of the unit ranges from
175 to 500 meters.
VOLCANIC ROCKS OF UPPER TOKLAT RIVER
Three t o 5 kilometers south of Polychrome Mountain
along the east branch of the Toklat River are two eastdipping volcanic units (pl. 1, fig. 5). The lower unit is
composed of interbedded andesite and rhyolite flows
that conformably(?) overlie the Cantwell Formation; the
upper unit is predominantly rhyolite.
The lower andesite-rhyolite unit is 1,350 meters
thick and contains alternating andesite and rhyolite
flows from 5 to 20 meters thick. The andesite flows are
highly vesicular and often contain geodes. Some of the
rhyolite flows may be devitrified welded rhyolite tuff
with microphenocrysts of quartz, sanidine, and biotite.
East of and above the andesite-rhyolite unit is a
series of porphyro-aphanitic rhyolite flows. This unit
has a thickness of at least 555 meters. Scattered exposures suggest that these flows merge with rhyolite
units in the upper Teklanika River area.
INTRUSIVE ROCKS
Most of the intrusive rocks in the area are mafic and
intermediate sills, diltes, and small plutonic bodies (pl.
1). Intruding unit A of the upper Teklanika sequence is
a small mafic plug. On Cathedral Mountain about 0.5
kilometers east of the mouth of Tattler Creek is a fineto medium-grained quartz-bearing diorite; a diabase
plug and a series of fine-grained augite diabase sills
Figure 4. Looking north at eastern part of Polychrome Mountain. Tva - andesite and basalt flows, Tvf - rhyolite flows,
Tc - Cantwell Formation.
THE TEKLANIKA FORMATION
7
Figure 5. Looking south at east-dipping andesite and rhyolite flows on east side of upper Toklat River.
are exposed along the east side of the mountain (fig. 6).
A small fine- to medium-grained quartz-bearing diorite plug covering 0.15 square kilometers is found on
Polychrome Mountain along the Denali Highway, and an
east-west-trending diabase dike is exposed for 3 kilometers on the north side of the mountain. Numerous
mafic dikes and sills, ranging from 1t o 1 0 meters thick,
are found throughout the Cantwell and Teklanika
Formations. They are generally aphanitic and often
exhibit a directive fabric due to flow.
Felsic intrusive rocks are less common than mafic
intrusive rocks in the region, but rhyolite to quartz
latite dikes, sills, and small plugs crop out throughout
the area of the Teklanika Formation. A 25-meter-thick
porphyro-aphanitic rhyolite dike cuts unit A on the east
side of the upper Teklanika River. On the western side
of Polychrome Mountain, a rhyolite sill 1 4 meters
thick has intruded the Cantwell Formation. Three
hypabyssal porphyro-aphanitic rhyodacite to quartz
latite plugs are present near the mouth of Tattler Creek.
Most of these intrusive rocks were probably feeders
t o the volcanic rocks of the Teklanika Formation. The
intrusions are widely scattered, suggesting that the stratigraphically heterogeneous Teklanika Formation was
formed around many centers of eruption.
PETROLOGY
FELSIC FLOWS AND INTRUSIONS
Most of the felsic volcanic flows are porphyritic
rhyolites (table 1). The rocks have a microcrystalline t o
hyaline groundmass and a faint planar fabric caused by
either segregation of mafic and siliceous minerals or by
the alignment of feldspars.
Figure 6. Diabase plug on east side of Cathedral Mountain.
In the rhyolite flows, high sanidine (2V=0.40°)
occurs as euhedral to subhedral phenocrysts and microphenocrysts (fig. 7), and may contain simple Carlsbad
GEOLOGIC REPORT 47
Table 1.Modes of selected rhyolite flows and felsic intrusions (based on 300 points counted
per thin section and estimation of groundmass composition)-in percent.
Rock
NO.^
Location2 Quartz Sanidine
Glass
Iron
Plagioclase Biotite oxide
'(1) = Geochemical analysis number; refers to preceding sample
' ~ e= Upper Teklanika sequence
CI =Cathedral-Igloo Mountain area
P = Polychrome Mountain area
TO = Upper Toklat River area
Tr = Trace amount
3z = Zircon
a = Apatite
s = Sphene
g = Garnet
4~rimarilycalcite, sericite, and kaolinite
5~ncludestridy mite
twins. Plagioclase (An content not optically determinable) is found as a minor subhedral microphenocryst
which may be surrounded by cumulophyric sanidine.
Quartz rarely occurs as phenocrysts, but is common as
a late interstitial constituent. Tridymite and cristobalite
commonly fill vesicles. Iron oxide minerals are abundant. Microcrystalline (0.01 t o 0.2 mm) minerals include subhedral t o euhedral sanidine and interstitial
quartz, and primary spherulites of microcrystalline
quartz, sanidine, and cristobalite surrounded by limonite are common (fig. 8). Plagioclase is occasionally
present in the groundmass. Zircon and rarely apatite
occur in trace amounts. Secondary minerals include
kaolinite, calcite, and biotite; kaolinite, wholly or in
part, commonly replaces groundmass and sanidine phenocrysts.
Trace
accessory
minerals3
Limonite
Alteration
minerals4
Petrology and texture of nearby felsic intrusive
rocks strongly suggest they are related to the rhyolite
flows (table 1).The rhyolite dike in unit A of the upper
Teklanika sequence and the rhyolite sill on the west
side of Polychrome Mountain are hypidiomorphic and
holocrystalline. Phenocrysts of anhedral fine-grained
quartz, fine-grained biotite, and fine- to medium-grained
anhedral t o euhedral high sanidine occur in a groundmass of sanidine, quartz, and limonite. Biotite commonly has a reaction rim containing magnetite, and
deuteric alteration of sanidine phenocrysts and groundmass to sericite and kaolinite is common. The rhyolite
dike and sill are similar in texture and mineralogy t o the
rhyolite flows in the Teklanika Formation.
Three small hypabyssal quartz latite plugs near the
mouth of Tattler Creek may either be related t o
volcanic flows or represent a separate igneous event.
In these rocks, fine- to medium-grained subhedral
labradorite phenocrysts and fine- to medium-grained
subhedral sanidine phenocrysts are set in a rnicrocrystalline to fine-grained groundmass of sanidine, quartz,
and biotite. Apatite, zircon, and sphene are common
accessory minerals. Trace amounts of anhedral garnet
are found in the, plugs. If these rocks are derived from
the same magma as nearby flows, they probably are
the result of a mixing of andesite and rhyolite magmas.
THE TEKLANIKA FORMATION
Figure 7. Photomicrograph of rhyolite flow from unit
F of upper Teklanika sequence. Phenocrysts of
quartz and sanidine are set in a quartz-cristobalitesanidine groundmass. Cristobalite forms the dark
halo around the resorbed quartz phenocryst. q
quartz, s - sanidine. Plane polarized light.
PYROCLASTIC ROCKS
Pyroclastic rocks are most commonly found interbedded with felsic flows and rarely with mafic flows.
They are dominantly a combination of andesite to
rhyolite vitric, lithic, and crystal welded tuffs, Some
graded volcaniclastic sandstone beds containing tuffaceous fragments suggest deposition in a quiet aquatic
environment.
Welded tuffs contain either crystals or lithic fragments, or both, in a devitrified groundmass of shards
and pumice fragments (figs. 9 and 10). Crystal fragments include high sanidine (2V=30-35O), calcic plagioclase (An 3 6 - ~ ~and
) rarely, quartz. ~ a n i d i n e and
plagioclase are commonly found in the same rock.
Opaque oxides are common accessory minerals in the
groundmass. Glass shards have been contorted and
flattened, and range from 0.1 to 0.5 mm in length.
The shards commonly display axialitic or spherulitic
textures due to devitrification to cristobalite. A wide
variety of cognate lithic fragments are derived from
flows or tuffaceous rocks; they range from 0.05 to
0.08 mm in length.
MAFIC AND INTERMEDIATE FLOWS
AND INTRUSIONS
Mafic flows range from olivine basalt to andesite,
but most flows are andesites (table 2). These rocks are
Figure 8. Photomicrograph of rhyolite f l ~ wfrom Cathedral Mountain. Groundmass of quartz-feldspar
spherulites and limonite exhibits faint flow foliation.
s - sanidine phenocryst. Plane polarized light.
hgpidiomorphic and holocrystalline (figs. 11 and 12).
Plagioclase (An 42-60) is generally subhedral, tabular,
and twinned, and exhibits resorbed crystal outlines.
Complex zoning, with calcic plagioclase cores, is common. Minor interstitial sanidine is occasionally present.
Augite (2V=30-40°) is commonly ophitic, anhedral, and
) a common
fine to medium grained. Olivine (Fo 6 0 - 6 ~ is
fine-grained constituent and occasionally forms mediumgrained phenocrysts. Alteration of the olivine to serpentine and iddingsite is ubiquitous. Ilmenite and
magnetite are usually present,-and apatite and zircon
are frequent accessory minerals. A few basalt and
andesite flows on Cathedral Mountain exhibit a directive fabric'due to the orientation of the plagioclase
crystals. Dense magnetite-rich (36-39%) basalt flows
are a minor part of unit E in the upper Teklanika
sequence. These rocks also contain fine-grained plagioclase, olivine, and augite. Secondary minerals observed
in the basalt and andesite flows are chiefly sericite,
calcite, and biotite.
Numerous mafic and intermediate sills, dikes, and
small plutons cut both the Teklanika Formation and
the nearby Cantwell Formation. On the southwest
side of Cathedral Mountain, a small fine- to mediumgrained quartz-bearing diorite plug contains subhedral
tabular andesine and medium-grained subhedral sanidine
(2V=35O). Augite is the only primary mafic silicate
present, and hydrothermal alteration has introduced
9
10
GEOLOGIC REPORT 47
Figure 9. Photomicrograph of devitrified welded tuff
from Cathedral Mountain displaying highly compressed shards and sanidine crystal fragments. s sanidine, q - quartz. Crossed nicols.
calcite and biotite.
On the east side of Cathedral Mountain are a finegrained hypidiomorphic and holocrystalline diabase plug
and numerous sills. The plug and sills contain subhedral
labradorite, anhedral augite, and ilmenite. Magnetite
and apatite are very fine-grained accessories, and secondary biotite has been hydrothermally introduced.
A quartz diorite plug on Polychrome Mountain is
holocrystalline, fine to medium grained, inequigranular,
and hypidiomorphic. The plug contains medium-grained
subhedral andesine (An 3 8 - 4 , interstitial alkali feldspar
(surrounding the plagioclase grains), anhedral olivine
(Fo 45-50), anhedral medium-grained augite, and subhedral to euhedral quartz. Apatite is fine grained,
euhedral, equant t o prismatic, and makes up 2 t o 4
percent of the plug. Accessory minerals include magnetite, ilmenite, hematite, zircon, and garnet.
An east-west-trending olivine-hornblende gabbro dike
crops out on the northern edge of Polychrome Mountain. It contains fine- to medium-grained subhedral
tabular labradorite. Hornblende is completely replaced
by magnetite, leaving relict fine- to medium-grained
subhedral to euhedral hornblende outlines. Augite is
subhedral to euhedral, cumulophyric, and zoned. Olivine
is both fine grained and anhedral and medium-grained,
euhedral, and zoned.
Many other andesite and basalt dikes cut the volcanic
flows and the Cantwell Formation. The dikes are dark
gray, aphanitic, and equigranular. Plagioclase accounts
for about 70 percent of the rocks, with the rest being
mainly limonite and opaque oxides. The dikes are
Figure 10. Photomicrograph of devitrified welded tuff
along lower Calico Creek. Extreme-compression and
stretching has nearly eliminated shard texture. r - rhyolite rock fragment, s - sanidine fragment, q - quartz.
Plane polarized light.
commonly partially altered to calcite, sericite, and
chlorite. Both geologic setting and petrology suggest
that the andesite and basalt dikes and sills, together with
the other mafic and intermediate intrusions discussed,
have direct extrusive equivalents.
GEOCHEMISTRY
Chemical data from 11 samples suggest that the
Teklanika Formation forms a calc-alkaline volcanic
suite as is commonly found in continental margin
volcanic belts (Green and Ringwood, 1968; Jakes and
White, 1972). This conclusion is based on the chemical
classification of Irvine and Baragar (1971), whereby
volcanic rocks are classified into three main types:
subalkaline, alkaline, and peralkaline, with subalkaline
rocks being subdivided into a tholeiitic basalt series and
a calc-alkali series. Chemical analyses and CIPW norm
values for the 11 samples are given in table 3. Four of
the samples were modally classified as rhyolite, four as
andesites, and one as a basalt; one sample is from a
quartz-latite plug and one is from a quartz-bearing
diorite pluton. A chemical classification of the nine
volcanic samples analyzed according to the method of
Irvine and Barager (1971) is given in figure 13.
Chemically, the volcanic rocks of the Teklanika
Formation are subalkaline, as suggested by the presence
of normative quartz, and hypersthene and by their
position on the alkalies-silica diagram (fig. 14). Furthermore, an AFM plot (fig. 15), which shows a lack of
Table 2. Modes o f selected andesite and basalt flows and mafic and intermediate intrusions
(based on 300point count)-in percent.
Rock No.'
~ o c a t i o n Qbartz
~
Sanidine
---
--
Te
Te
Te
Te
Te
Te
Te
-------
--
2
Te
Te
Te
----
---
.i
Te
CI
CI
CI
CI
---
---
0.6
--
DE14-2
DE14-6
EA14-36
CC15-5
CC15-6
RE15-30
RE15-38
(6)
, 74WG-4A
74WG-4B
74WG-6
2
(7)
% 74WG-14
P
JR12-43
9 RJ13-48
3 RJ13-51
SL13-44A
a
(8)
SL13-44B
DT73-9
(9)
M73-14
TKB2-33
(10)
TKB2-57
M73-36B
EH5-2A
EH5-2B
4
-
---
--
---
0.9 a
Tra
--
--
-----
0.3
CI
CI
--
--
CI
P
---
--
P
P
TO
To
---
---
-
2
~ ~ 1 3 - 6 CI
.gSL13-37
CI
2 DT73-5
CL
2 M73-21A CI
a~ M73-23
CI
Y
.2 TBK2-12
P
.d
2
-
---
--
--
-- -
5.9
--
2.9
0.3
5.5
3.0
6.9
---
--6.1
2
2
TKB2-35
EH3-1
5 EH3-11
u 554-34
3 Dz73-1
E
(11)
M73-38
P
P
P
P
P
P
Augite
Accessory
minerals3 Serpentine Iddingsite Limonite
0,3 a
0.7 a
-Trz
0.3 a
--
1k
Plagioclase Hornblende
Iron
Olivine oxides
2.6
8.4
--
--
---
--
--
2.7
--
3.7
--
' ( 7 ) Geochemical analysis number.
' ~ e= Upper Teklanika sequence
CI = Cathedral-Igloo Mountain area
P = Polychrome Mountain area
To = Upper Toklat Fiver area
3, = Zircon
a = Apatite
s = Sphene
g = Garnet
R = Trace
h m a r i l y calcite, chlorite, and kaolinite.
Tra
0.3 a
--
0.3 a
0.3 a
Tr g,
3.7 a,
Trz
1.6 a
0.3 a
5.4
12.0
14.1
1.6
3.6
3.7
Other
alteration
minerals4
12
GEOLOGIC REPORT 47
E
basalt
tholriltio
andesitc
80
60
+- Normative
20
40
0
Plagioclase Composition
Figure 13. Chemical classification of nine volcanic rocks
from Teklanika Formation after Irvine and Baragar,
1971.
Figure 11. Photomicrograph of andesite flow from
Polychrome Mountain. p - plagioclase, a - augite,
b - biotite. Crossed nicols.
O
lo[
V
Figure 12. Photomicrograph of andesite flow from unit
E of upper Teklanika sequence. p - plagioclase,
a - augite, o - olivine altering to serpentine and
iddingsite. Crossed nicols.
iron enrichment for low alkali values, and a plot of
A1203 versus normative plagioclase (fig. 16) strongly
suggest that the rocks are part of a calc-alkali series
rather than a tholeiitic basalt series, although the A1203
content of basalts and andesites is somewhat transitional
Subalkaline
Alkaline
-...-
Mid-Tertiary
Early Tertiary
Late Cretaceous
Jurassic
Figure 14. Alkalies-silica diagram with nine volcanic
rocks from Teklanika Formation plotted (solid circles - andesite and basalt; open circles - rhyolite).
Solid line separates alkaline and subalkaline rocks
after Irvine and Baragar, 1971. Broken lines enclose
chemical trends of plutonic rocks from AlaskaAleutian Range batholith from Reed and Lanphere,
1973.
between the two series. The presence of modal augite,
quartz, and phenocrysts of pyroxene, olivine, and
plagioclase are also consistent with, mineral phases
commonly found in calc-alkaline basalts and andesites
(Hyndman, 1972).
Because of the generally siliceous nature, low Na-K
ratio, lack of iron enrichment, and trace amounts of
THE TERLANIKA FORMATION
13
Table 3. Geochemical analyses (rapid rock technique by Skyline Labs, Wheatridge, COf
and ClPW norm calculations-in percent.
-
Sample1
Oxides
1
2
3
4
5
6
7
8
9
10
11
Si02
Ti02
A1203
Pe203
FeO
MnO
MgO
CaO
Na20
K20
'2O5
Norms
Quartz
Orthoclase
Albite
Anorthite
Corundum
Diopside
Hypersthene
en2
fs3
Olivene
Magnetite
Ilmenite
Apatite
Calcite
Hematite
Rutile
'~eochemical analysis number (see tables 1 and 2).
i~nstatite
Ferrosilite
Table 4. Analytical data for K-Ar age determinations.
Sample
Rock type
Mineral
da Led
DT-73-9
DT-73-1
74 WG-14
andesite
quartz diorite
basalt
plagioclase
plagioclase
whole rock
K20
(weight
percent)
Sample
weight
(gm)
4
0
X 10-11
0.600, 0.610
0.320, 0.31 7
2.30, 2.30
4.4935
3.6977
1.9480
24.73
2.74
14.36
~
mole^?&)
40
~A r r a d
4 0 ~
X 10-2
0.3600
0.3400
0.2472
40Arrad
-
0.812
0.661
0.480
age^ 2 6
60.6 Min.
57.2 rt3.4
41.8 Min.
Constants used in age calculations:
XE = 0.585 x 1 0 - ~ 0 / y r
- 4.72 x 10-10/yr
K 4 4 i total = 1.19 x 10-4 mole/rnole
garnet, the Teklanika Formation may have been produced by eclogite-controlled fractionation at a depth
greater than 1 0 0 kilometers (Ringwood, 1974). The
bimodal character of the formation may be caused by
contemporaneous fractionation in the mantle of hydrous
parental material into two contrasting magmas as suggested by Yoder (1973).
Figure 1 4 also reproduces the chemical trends of 139
GEOLOGIC REPORT 47
A (K20 + Na20)
*
F ( ~ e 00.8998 Fe,03)
M (MgO)
T holeiitlc field
Calc-alkaline field
M
Figure 15. AFM diagram with nine volcanic rocks from Teklanika Formation plotted. Broken line separates tholeiitic
and calc-alkaline rocks after Irvine and Baragar, 1971.
plutonic rock samples from the Alaska-Aleutian Range
batholith published by Reed and Lanphere (1973).
The diagram vaguely suggests that ;he rhyolite flows of
the Paleocene Teklanika Formation may be cogenetic
with early Tertiary granitic rocks of equivalent age in
south-central Alaska, although more petrologic and
chemical studies are needed.
GEOCHRONOLOGY
Radiometric ages of two volcanic and one related
plutonic rock were determined by the potassium-argon
method (analytical data reported in table 4; see Turner
and others, 1973, for discussion of analytical method
used). Plagioclase from the andesite flow on Cathedral
Mountain (DT-73-9) yielded a minimum age of 60.6
m.y. A whole-rock sample from a basalt flow in the
upper East Fork drainage yielded a minimum age
of 41.8 m.y. These two dates must be considered
minimum ages because incipient alteration is likely to
have caused postcrystallization argon loss. Plagioclase
from the quartz-diorite plug on Polychrome Mountain
(DT-73-1) yielded an age of 57.2 L 3 . 4 m.y.
Five whole-rock potassium-argon ages from the Teklanika Formation to the east (Hickman, 1974) range
2.1 m.y. to 59.3 + 2.7 m.y. Alteration is
from 49.5
reported from these rocks also, and their ages must be
considered minimum ages for Teklanika Formation
volcanism.
All these dates suggest that eruption of the Teklanika
Formation took place during Paleocene ltme. Because
the Teklanika Formation is closely associated in time
+
15
THE TEKLANIKA FORMATION
and space with the underlying Cantwell Formation, the
radiometric ages give a minimum age for the deposition of the Cantwell Formation as well. The age dates
agree with paleobotanical dating of the Cantwell Formation (Wolfe and Wahrhaftig, 1970; Wolfe, 1972),
which indicates that the formation is Paleocene in age;
they also agree with a maximum age of the Cantwell
Formation t o the east o f 9 5 m.y. (Wahrhaftig and
others, 1975). Thus, a closely spaced series of events,
beginning with deposition of the Cantwell Formation
and ending with orogeny and its associated calc-alkaline
volcanism (Gilbert, 1975), occurred during Paleocene
time in the central Alaska range.
ACKNOWLEDGMENTS
The authors wish t o thank the National Park Service,
in particular Daniel Kuehn and Vernon Ruesch, superintendents a t Mount McKinley National Park, for their
cooperation with o u r field studies.
We also appreciate the field assistance of the following members of the 1 9 7 2 and 1 9 7 3 University of Alaska
geology department camps: Edward Anderson, Enayat
Aziz, Thomas K. Bundtzen, Charles Collins, Floyd
Demman, Richard Droker, Ronald Edquist, Dennis
Erickson, Carmen Fischer, Robert Gaddis, Richard
Jirik, Steve Johnson, Steve Lowell, James A. Madonna,
Patricia Potts, Earl Redman, William Roberts, Mark
Robinson, James Robar, Ernest Sloan, and Mark Zdepski.
In addition, special thanks are due t o Bundtzen,
Robert B. Forbes, and Florence R. Weber for helpful
discussions and criticism, and t o John W. Buza for
assistance during the 1 9 7 4 field season.
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16
GEOLOGIC REPORT 47
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